EP 460 824 A1 and EP 0 863 309 A1 have each disclosed electric machines which serve as starter devices for internal combustion engines. For the purposes of varying the rotational speed and torque characteristic of the electric motor provided there as a drive for driving the drive element, which is preferably in the form of a pinion, a planetary gearing is disclosed. Said planetary gearing has an internal gear which is mounted in a housing part, the latter being in the form of a drive bearing. For this purpose, an engagement element of the internal gear engages into an engagement element of the housing or drive bearing.
Provision is made for vibrations that are generated in the internal gear to be transmitted directly to a component which, with regard to acoustics, has expedient airborne noise radiation and damping characteristics. The housing or drive bearing has these characteristics owing to its geometry, which has a multiplicity of changes in cross section and, in association therewith, a multiplicity of step changes in stiffness, and small radiating surfaces.
Low noise, which is acceptable with regard to acoustic impression, in the motor vehicle is of increasing importance. This applies even in the case of a starting process which is completed after only a few seconds. Start-stop applications and, in future, also the so-called “sailing” mode additionally intensify the requirements and demand corresponding acoustically optimized starters. The so-called “sailing” mode refers, in technical terms, to a method in which, during travel on the road, the internal combustion engine is deactivated when it does not need to transmit any drive power, and is reactivated only when drive power has to be transmitted. Such a driving state exists for example when traveling downhill, such that the transmission of a drive torque or drive power becomes necessary, and thus a starting process is rendered necessary during travel, only after a transition for example to travel on a level road or even an uphill road again.
One of the main noise sources in the starter is the planetary gearing, which is commonly designed as an epicyclic gearing. Owing to alternating tooth meshing and, as a result, fluctuations in rigidity, adjacent components are subjected to excitation and are thus caused to vibrate. Some of said vibrations are radiated by said components as airborne noise, or are transmitted to other surrounding starter components or even engine components, that is to say components of the internal combustion engine, as body-borne noise. In the case of the abovementioned gearing designs, those components whose geometric design exhibits only small changes in cross section and low natural frequencies and large radiating surfaces are subjected to direct excitation. Such components therefore have an unfavorable vibration characteristic with regard to the reduction of noise. In particular, the direct connection of the internal gear to the pole housing, the intermediate bearing and the bearing cover are acoustically unfavorable.
Provision is made for the internal gear, as a noise source, to be fastened in the starter such that as few components as possible are subjected to excitation. The required torque support should in this case be realized on asymmetrical components with good damping characteristics. Excitation of adjacent components should be prevented or reduced.